1. Field of the Invention
This invention relates generally to methods for the electrochemical digital etching of compound semiconductors comprising alternating layers of a first element and a second element, and relates specifically to a method for the cyclical removal of a bilayer of the compound semiconductor by removing an atomic layer of the first element at a first potential and then removing an atomic layer of the second element at a second potential. The process of the invention can be applied to compound semiconductors produced by the electrochemical atomic layer epitaxy (ECALE) which is the subject matter of copending application Ser. No. 07/695,969, filed on May 6, 1991, now U.S. Pat. No. 5,320,736, which is incorporated herein by this reference, and also can be applied to any compound semiconductor comprising alternating layers of the constituent elements.
2. Prior Art
The trend towards low dimensional materials in the field of electro-optics has resulted in the need for atomic level control of deposit structure and dimensions. The family of techniques referred to as Atomic Layer Epitaxy (ALE) has arisen to help address that need. Finnish Patent No. 52359, U.S. Pat. No. 4,058,430; Goodman, C. H. L. et al., J. Appl. Phys. 60, R65 (1986); Kuech, T. F. et al., Atomic Layer Growth and Processing, Mater. Res. Soc. Proc. 222, Pittsburgh, Pa., 1991. The principle of ALE is that atomic layers of individual elements are deposited sequentially. In this way, layer-by-layer growth is maintained and deposit thickness is controlled by the number of deposition cycles, not by the reactant flux.
The present principal inventor has been working on the electrochemical analogue of ALE, referred to as Electrochemical Atomic Layer Epitaxy (ECALE). Stickney, J. L. et al., J. Electroanal. Chem., 300 543 (1991); Stickney, J. L. et al., J. Electrochem. Soc., 138, 1279 (1991); Stickney, J. L. et at., Atomic Layer Growth and Processing, edited by Kuech, T. F. et al., Mater. Res. Soc. Proc. 222, Pittsburgh, Pa., 1991, p. 283. ECALE is a method for the formation of compound semiconductors where individual elements are alternately electrodeposited at underpotential from separate solutions. The term underpotential refers to the fact that compound formation is generally energetically favorable, compared with formation of bulk deposits of the individual elements. Kolb, D. M., Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, Eds. Gerischer, H. et al., John Wiley; New York, 1978, p. 125; Juttner, K. et al., Z. Phys. Chem. N.R., 122, 163 (1980). Thus, electrodeposition of one element on a second, forming a surface compound, will frequently occur at a potential prior to (under) that required to deposit the bulk element. Deposition is surface limited; limited to an atomic layer, as diffusion of the depositing element into the substrate is generally a slow process at room temperature.
In addition to atomic control over deposition, atomic control over material removal is desirable. Digital etching is just beginning to emerge as an active area of research. Present attempts at digital etching involve cycles where a reactant gas is first absorbed on the substrate surface and then a reaction is initiated using an energetic beam of electrons, ions or photons. Reaction of the monolayer of adsorbed gas occurs only with the surface, forming a volatile compound, and thereby removing one atomic layer of the substrate. Etching involves the repeated application of this cycle. Digital etching is applicable to elemental semiconductors, compound semiconductors and other materials such as metals and insulators.
The above studies involved a single etchant gas and a subsequent reaction initiation. Meguro, T. et al., Appl. Phys. Lett. 56, 1552 (1990); Horiike, Y. et al., J. Vac. Sci. Technol. A, 8, 1844 (1990); Meguro T. et al., Atomic Layer Growth and Processing, edited by Kuech, T. F. et al. Mater. Res. Soc. Proc. 222, Pittsburgh, Pa., 1991, p. 121; Sakaue, H. et al., Atomic Layer Growth and Processing, edited by Kuech, T. F. et al. Mater. Res. Soc. Proc. 222, Pittsburgh, Pa., 1991, p. 195; Maki P. A. et al., Appl. Phys. Lett., 55, 91 (1989). ALE of compound semiconductors involves the use of different reactant precursors for the deposition of the individual elements. Finnish Patent No. 52359, U.S. Pat. No. 4,058,430, Goodman C. H. L. et al., J. Appl. Phys. 60, R65 (1986); Kuech, T. F. et al., Atomic Layer Growth and Processing, Mater. Res. Soc. Proc. 222, Pittsburgh, Pa., 1991. This suggests scenarios for digital etching of compound semiconductors where atomic layers of each of the component elements are removed preferentially using separate etchant species, leaving an atomic layer of the other element. The present specification describes performing digital etching electrochemically on compound semiconductors where electrons are used to remove an atomic layer of one element, and holes are used to remove an atomic layer of the other. This process is a natural outgrowth of ECALE, as again, the underpotentials for the component elements are used, although, in the digital etching process a single electrolyte solution is used, containing no precursors to the substrate's component elements.